Arab Board Radiology Part 1 key terms

A calibrated scale expressing the linear attenuation coefficient of a tissue relative to water, where water is 0 and air is about -1000. It allows objective comparison of tissue density on CT images.

Display settings that map a chosen range of CT attenuation values onto the available grey scale. Level sets the midpoint brightness and width sets the contrast span, allowing soft tissue or bone to be viewed optimally.

Dark streaks or cupping produced when a polychromatic X-ray beam preferentially loses low-energy photons as it traverses tissue. It is most prominent between dense structures such as bone or metal.

Absorption of an X-ray photon by an inner-shell electron, which is ejected while the photon disappears entirely. Its probability rises steeply with atomic number and falls with increasing photon energy, driving bone and contrast visibility.

Interaction in which a photon transfers part of its energy to an outer-shell electron and is deflected in a new direction. It degrades image contrast and is the main source of occupational radiation exposure in fluoroscopy.

A sudden increase in photoelectric absorption that occurs when photon energy just exceeds the binding energy of an atom's innermost shell. It explains why iodine and barium provide strong contrast at diagnostic energies.

The thickness of a specified material that reduces the intensity of an X-ray beam to half its original value. It is a standard measure of beam quality and filtration adequacy.

The practice of keeping radiation doses as low as reasonably achievable, taking economic and social factors into account. It underpins protocol optimisation, shielding and dose monitoring in every imaging modality.

A whole-body dose equivalent that reflects the relative radiosensitivity of exposed organs, expressed in sieverts. It allows comparison of stochastic risk between different examinations and modalities.

Stochastic effects, such as cancer induction, have no threshold and their probability rises with dose. Deterministic effects, such as skin erythema, occur only above a threshold and increase in severity with dose.

A radiation protection assumption that cancer risk increases linearly with dose from zero, with no safe threshold. It is used conservatively for setting dose limits despite uncertainty at very low doses.

The total radiation dose delivered to an organ or tissue over 50 years after intake of a radionuclide. It accounts for the biological half-life and decay of the incorporated isotope.

A device containing a long-lived parent isotope that continuously produces a shorter-lived daughter, which is eluted for imaging. The technetium-99m generator, using molybdenum-99, is the commonest example.

An imaging technique that detects pairs of annihilation photons emitted when positrons from a radiotracer meet electrons. It maps metabolic activity, most often with fluorine-18 fluorodeoxyglucose.

A semi-quantitative measure of radiotracer accumulation in a lesion, normalised to injected activity and body size. It is used to grade malignancy and assess treatment response on PET studies.

A water-soluble compound containing iodine that attenuates X-rays, used to opacify vessels and organs. Modern agents are usually low-osmolar monomers or dimers and carry risks of allergy-like and renal reactions.

An acute rise in serum creatinine occurring within days of intravascular iodinated contrast administration, after other causes are excluded. Risk is greatest with pre-existing renal impairment, diabetes and dehydration.

A non-immunological, allergy-like adverse reaction to contrast medium that does not require prior sensitisation. It ranges from urticaria to bronchospasm and hypotension and is managed with antihistamines, steroids and adrenaline.

A paramagnetic chelate that shortens T1 relaxation and brightens tissues on MRI. It is avoided in severe renal failure because of the risk of nephrogenic systemic fibrosis.

A rare fibrosing disorder of skin and internal organs linked to gadolinium exposure in patients with severely impaired renal function. Risk is minimised by screening renal function and using stable macrocyclic agents.

Time constants describing how nuclear magnetisation recovers along the main field (T1) and decays in the transverse plane (T2) after a radiofrequency pulse. Tissue differences in these times generate MRI contrast.

An MRI pulse sequence that refocuses signal by reversing magnetic field gradients rather than using a 180-degree radiofrequency pulse. It permits fast imaging and is sensitive to susceptibility and flow.

Local signal loss and geometric distortion caused by differences in magnetic susceptibility between tissues, notably near metal or air. It is exaggerated on gradient echo sequences and can obscure adjacent structures.

Scatter radiation consists of deflected photons that blur the image and increase staff dose. An anti-scatter grid of alternating lead and spacer strips absorbs much of it before it reaches the detector.

Filtration removes low-energy photons from the beam before the patient, reducing skin dose. Collimation restricts the field to the region of interest, reducing patient dose and scatter production.

A dose benchmark, usually set at the third quartile of a survey of typical practice, used to flag examinations with unusually high patient doses. Exceeding it prompts protocol review rather than a legal breach.

PACS is the networked archive that stores and distributes medical images. DICOM is the international standard governing image file format and communication between scanners, workstations and archives.

The level of contrast, resolution and noise sufficient to answer the clinical question at the lowest reasonable dose. Optimisation balances these factors rather than maximising any single one.

A measure of how faithfully an imaging system reproduces spatial frequencies, describing resolution performance. It falls off at high frequencies, indicating the limit of detectable detail.

A delayed MRI phase, typically 20 minutes after a hepatocyte-specific gadolinium agent, in which functioning liver takes up contrast and appears bright while most lesions appear dark. It improves detection of hepatic lesions.

The layered enhancement pattern of normal small bowel on contrast-enhanced CT, reflecting mucosa, submucosa and muscularis. Loss of stratification suggests transmural inflammation, ischaemia or tumour.

A calculated map derived from diffusion-weighted imaging that quantifies water molecule mobility in tissue. Low values indicate restricted diffusion, seen in infarction, abscess and highly cellular tumours.

Evidence-based recommendations for managing incidentally detected pulmonary nodules, based on size, morphology and patient risk. They define when to follow up with CT, when to use PET, and when to biopsy or stop surveillance.

A standardised lexicon and reporting system for breast imaging that assigns a final assessment category from 0 to 6. Each category carries a defined likelihood of malignancy and a recommended management pathway.

A breast imaging technique that acquires multiple low-dose projections over an arc and reconstructs thin slices through the breast. It reduces tissue overlap and improves cancer detection in dense breasts.

A method of gaining vascular or cavity access by puncturing with a needle, passing a guidewire, removing the needle and then advancing a catheter over the wire. It is the foundation of most interventional procedures.

Deliberate occlusion of a blood vessel by injecting particles, coils, glue or balloons through a catheter. It is used to control bleeding, treat vascular malformations and devascularise tumours.

Delivery of chemotherapy directly into a tumour's arterial supply followed by embolisation of those vessels. It is used mainly for intermediate-stage hepatocellular carcinoma, exploiting the tumour's arterial dominance.